Small Bodies in the Solar System Our Solar System consists of the Sun, of the well known planets with their natural satellites (moons), and of a large number of lesser bodies which are also orbiting the Sun - they range from tiny dust specks to asteroids like Ceres with diameters of almost 1000 km. All these objects were formed at the origin of the Solar System, about 4.6 billion years ago. To be more precise, they were built up from tiny chunks of interstellar matter, present in the huge disk out of which the Solar System was formed. There was enough matter to allow the formation (by soft collisions, electrostatic attraction and gravitational forces) of small bodies of a few tens, hundreds or thousands of metres, as far as 100 times farther away from the young Sun (about 15 billion km) than our Earth (150 million km). The interstellar matter consisted of gaseous molecules (mainly hydrogen and helium), but also of solid particles (such as silicates), coated by ices (typically of water and carbon dioxide), that could be processed into bigger carbon-rich molecules by the effects of cosmic radiation. Comets and asteroids form at different distances In the young Solar System, as well as nowadays, the temperature decreased with increasing distance from the Sun. Therefore, just as everlasting snows can be seen high in the mountains, some interstellar ices could survive about five times farther away from the Sun than the Earth (about 750 million km), and also further away. This is the reason why the small bodies which were formed quite far away from the Sun were mixtures of dust and ice similar to huge dusty snowballs (the so-called cometary nuclei), whereas those that formed closer to the Sun were composed mainly of rock or metals (the so-called asteroids). The cosmic billiard game Small bodies which were formed at distances from the Sun less than about 30 times the distance of the Earth (4.5 billion km) were trapped in a kind of cosmic billiard game, under the attractions of the giant planets Jupiter, Saturn, Uranus and Neptune. Some of them were thrown into the inner Solar System, and, as we know from the samples that the Apollo astronauts have brought back from the Moon, eventually produced catastrophic impacts on the inner planets (Mercury, Venus, Earth and Mars) and on their satellites. Other small bodies were pushed to the outer edge of the Solar System, reaching distances about 100,000 times the distance of the Earth (15,000 billion km). This region is known as the Oort Cloud after Jaan Oort who studied the orbits of long-period comets in the early 1950's and found that some of them come from a distant reservoir of small objects.